#!/usr/bin/env perl
use strict;
use warnings;
use NetAddr::IP;
use feature 'say';

my $final;
my $input = '127.0.0.3';

my @ip = split /\./, $input;

if ($ip[3] % 2) {
    $final = join '.', @ip;
    $final = NetAddr::IP->new($final.'/8') + 1;
}
else {
    $final = join '.', @ip;
    $final = NetAddr::IP->new($final.'/8') - 1;
}

$final = substr $final, 0, -2;
say $final;

__END__

$ perl test.pl
127.0.0.4